Text appearing as "Not Delivered" but gets delivered anyway

[hazoolcat]

The red (!) bubble appears next to my text messages in iMessage, saying "Your message was not sent. Tap Try Again to send this message". However the recipient recieves my text as normal, nothing appears different on the recipients' device. I just get notifications that my message didn't send.

How do I fix this issue? Is it a problem with my SIM or my device? It has only been happening for a few weeks, and I've had this SIM for months.

[MI5]

@hazoolcat 

It's a problem with Apple and how O2 messaging works.

O2 network doesn't support delivery reports in the way Apple think, so you get a fail message.

To get an accurate report you need to start all texts with *0# and ignore the system message.